1. Field of the Invention
The present invention relates to a self-resetting overcurrent protection element using organic composition with PTC (positive temperature coefficient) characteristics.
2. Description of the Prior Art
Conventionally, a self-resetting overcurrent protection element using an organic composition with positive temperature characteristics has an element body with electrodes attached to opposite sides of the element body and a lead wire connected to each electrode. A sheathing material is wrapped around the structure with the lead wires extending therefrom. Epoxy resins, phenolic resins, or epoxidized phenolic resins are generally used for the sheathing material. All of these resins have high tensile stress capability when the elongation ratio is 10%, as well as an extremely low elongation ratio at the fracture point.
Another example of a self-resetting overcurrent element is described in Japanese Patent Publication No. 21601/1989. A positive characteristic thermistor is described wherein a case is used as the sheathing material. Lead wires having spring-like contacts are inserted into the case, connecting the lead wires to the electrodes. The spring tension exerted by the contacts on the electrodes also holds the element body and electrodes together.
A brief explanation of the principle of current limiting action of a self-resetting overcurrent protection element follows:
When an overcurrent occurs in a circuit using a self-resetting overcurrent protection element, the overcurrent protection element in the circuit generates Joule's heat. The heat causes the element body, which is made of polymer and conductive particles dispersed therein, to expand. As a result of this expansion, the conductive particles, which were dispersed in the element body and generally in contact with each other, separate, causing fewer particles to be in contact with each other. This causes the resistance of the element body to increase and current in the circuit to decrease, thereby limiting current in the circuit.
The efficiency and reliability of the current-limiting element are dependent upon the ability of its Positive Temperature Coefficient (hereinafter referred to as PTC) characteristics to maintain a high ratio of resistance between non-current limiting and current limiting situations.
When epoxy resin is used as the sheathing material in the first mentioned conventional overcurrent protection element, it exhibits high tensile stress when the elongation ratio is 10% during a tensile test and an extremely low elongation ratio at the time of fracture. This type of sheathing material presents the following problems: (1) A sheathing material with high tensile stress hinders the expansion of the element body at the time of current limiting action. Its low elongation ratio suppresses thermal expansion of the overcurrent protection element, thereby limiting the separation of conductive particles in contact with each other in the element body during an overcurrent condition. Consequently, the PTC characteristics of the element are restricted, thereby limiting the increase in resistance of the element at the time of current limiting action, and; (2) When thermal expansion of the element body exceeds a certain point, cracks occur in the sheathing material surface due to its low elongation ratio. As a result, the element body is exposed to outside atmosphere and its characteristics, such as, for example, voltage durability, deteriorate more rapidly than would otherwise occur.
The positive temperature characteristic thermistor described in Japanese Patent Publication No. 21601/1989 presents another problem in that the spring force exerted by the contacts of the lead wires on the electrodes and the element body suppresses adequate expansion of the PTC element body.